Hollow fiber membranes for a purpose of treatment of aqueous fluids have been widely utilized for industrial use such as microfiltration or ultrafiltration and for medical use such as hemodialysis, hemofiltration or hemodiafiltration. Particularly in recent years, there has been a demand for a technology for removing pathogenic substances such as virus from a solution of protein which is a useful ingredient during the steps for the manufacture of biopharmaceuticals and blood products so as to enhance the safety.
According to Non-Patent Document 1, it is said to be desirable, with regard to the steps of removal and inactivation of virus in a blood plasma protein fraction preparation, to incorporate not less than two different viral inactivation and removal steps. According to the description of Non-Patent Document 2, it is mentioned that the LRV to be achieved as the target value is about 4. Further, according to Non-Patent Document 3, there is a clear description in this document reading “Particularly with regard to the steps of removal and inactivation of virus, ‘it is desirable to consider not less than two different viral inactivation and removal steps’ in ‘Guideline for securing safety of blood plasma protein fraction preparations against virus’ (Dispatch No. 1047 for Drugs (Aug. 30, 1999)) and, with regard to the specific virus, it is requested that the sum of virus clearance indexes in the manufacturing steps (total virus clearance indexes) is 9 or more.” Incidentally, the above term LRV has nearly the same meaning as the virus clearance index R which is mentioned as follows in Non-Patent Document 1.Virus clearance index R=log((V1×T1)/(V2×T2))
V1 Volume before the treatment of the step
T1 Titer of virus before the treatment of the step
V2 Volume after the treatment of the step
T2 Titer of virus after the treatment of the step
As to a method for removal/inactivation of virus, there are a heating treatment, an optical treatment such as irradiation of gamma ray or ultraviolet ray, a chemical treatment such as a low-pH treatment, a precipitating fractionation such as fractionation by ethanol or fractionation by ammonium sulfate, a filtration by membrane, etc. and, in the removal of virus from a protein solution, a method of filtering by membrane which does not result in the denaturation of protein is attracting public attention.
On the other hand, in the steps for the manufacture of biopharmaceuticals and blood products, protein which is a useful ingredient should be efficiently permeated and recovered in view of the productivity. However, when the object for the separation and removal is a small-sized virus such as parvovirus, it has been difficult to simultaneously satisfy both of the removing characteristic for virus and the permeating characteristic for useful protein.
In Patent Document 1, there is a disclosure for a hydrophilic microporous membrane where the relation among an average permeability during 5 minutes from the start of filtration (globulin permeability A), an average permeability during 5 minutes since the stage being elapsed 55 minutes from the start of filtration (globulin permeability B), and the maximum pore size when 3 wt % bovine immunoglobulin where the percentage of the monomer is not less than 80 wt % is subjected to a low-pressure filtration at 0.3 MPa is expressed in terms of parameters. The constituent features of this membrane are as follows.
(1) Maximum pore size 10 to 100 nm
(2) Globulin permeability A>0.015×maximum pore size (nm)2.75 
(3) Globulin permeability B/globulin permeability A>0.2
Now, as mentioned in lines 21 to 27, page 3 of Patent Document 1, the requirement (1) merely mentions the pore size which is necessitated for the removal of infectious virus. The requirement (2) demands that the globulin permeability A is more than the value calculated from the maximum pore size of the micropore and, since it is obvious in a membrane for a purpose of removing the virus from a protein solution that the more the permeability for the protein solution, the better, it merely mentions the aimed characteristics. The requirement (3) demands that the permeability for a protein solution does not lower with elapse of time and that is also a mere description for the aimed characteristics which is demanded in the membrane where the removal of virus from a protein solution is a target. Besides the above, there are descriptions in subclaims for a hydrophilic microporous membrane where the logarithmic removal rate to porcine parvovirus is 3 or more, for a hydrophilic microporous membrane where the accumulated permeation amount during 3 hours from the start of filtration when 3 wt % bovine immunoglobulin where the ratio of the monomer is not less than 80 wt % is subjected to a low-pressure filtration at 0.3 MPa is not less than 50 liters/m2, etc. However, they merely mention the aimed characteristics of the membrane for a purpose of removal of virus from a protein solution that the virus is efficiently removed and the permeation amount of the protein solution is high. They do not give useful and specific information for an object to obtain a membrane having a high permeation of protein and a high removal of virus.
When (3) is considered in detail, when the ratio of the permeability after 55 minutes from the start of filtration to that immediately after the start of filtration merely becomes high, such a thing does not always consistent to the fact that permeability for the protein solution does not lower with elapse of time. For example, it is also thinkable the case where permeability for the protein solution gradually decreases with elapse of the filtration time but, at a certain point in time, defect is resulted in the membrane structure and, as a result, the permeability suddenly changes to increase. In that case, there may be also the case as a result that the permeability after 55 minutes from the start of filtration becomes high whereupon the ratio of the both exceeds 0.2. However, it cannot be absolutely said that a membrane showing the behavior as such achieves an object to obtain a membrane having a high permeation of protein and a high removal of virus.
Patent Document 1 discloses a microporous membrane which has a coarse and big structure layer having a big porosity and a dense layer having a small porosity. To begin with, the discussion made therein is for a hollow fiber membrane made of poly(vinylidene fluoride) (hereinafter, it will be abbreviated as PVDF) which is apt to form a homogeneous structure by means of thermally induced phase separation. Thus, it is difficult to directly apply such an art, for example, to a raw material such as a polysulfone type resin which has been widely used as a material for the hemodialysis membrane due to its high water permeation ability.
Patent Document 2 discloses a microporous membrane which has a coarse structure layer having a big porosity and a dense layer having a small porosity but, again, the thing predicted as a material herein is PVDF. PVDF is excellent in terms of physical strength but, on the other hand, since it is a hydrophobic material, it is apt to result in adsorption of protein, etc. and also in staining and blocking of the membrane whereupon the filtration rate quickly lowers. In order to improve such an undesirable characteristics, it is necessary to make the membrane hydrophilic but, generally, a membrane made from PVDF is to be modified to a hydrophilic one by means of post-treatment after preparing the membrane. Thus, as compared with the polysulfone resin where it is usual to make into the membrane in a blended state with hydrophilic polymer, there is a disadvantage that manufacturing steps become troublesome.
Patent Document 3 discloses an ultrafiltration membrane for retaining the virus having the initial LRV, to PhiX 174, of at least 4.0 and having a surface which has been made hydrophilic with hydroxyalkyl cellulose. In the art disclosed therein, hydrophilization is conducted by a specific hydrophilic polymer and is lacking in broad applicability. Although a blend of polysulfone, etc. with a hydrophilic polymer such as polyvinylpyrrolidone is also exemplified, a hydrophilizing treatment using hydroxyalkyl cellulose is still inevitable. Further, although a hollow fiber type is allowed as well, a flat sheet type is predicted and there is no sufficient explanation for preparing a hollow fiber membrane type.
Patent Document 4 discloses a method for the manufacture of an immunoglobulin preparation which effectively removes the virus and also does not cause hindrance for the filtration such as clogging of the removing membrane due to aggregates and contaminated protein in industrial production process. This method comprises a step where immunoglobulin solution is filtered using a porous membrane having an average pore size of 15 to 20 nm. There is a description reading that, as to a material for the porous membrane, regenerated cellulose is preferably exemplified. Further, in FIGS. 1 to 3, there are shown graphs where the accumulated filtrate amount spreads almost linearly against the elapsed time. It is of course thinkable that, when filtration is conducted using Planova 20N (Asahi Kasei Pharma) which is a virus-removing membrane made of regenerated cellulose mentioned in Example 1, such behavior will be noted. The above is greatly due to the affection by the use of a very highly hydrophilic regenerated cellulose material. In fact, it was very difficult to obtain a membrane showing such a linear filtration behavior in a synthetic membrane comprising hydrophobic polymer and hydrophilic polymer. In a cellulose membrane, strength in its wet state with water is low. Thus, there is such a disadvantage that it is difficult to set high filtering pressure whereby high permeability cannot be achieved.
Patent Document 5 discloses a macromolecular porous hollow fiber membrane having such a pore structure that, from the inner wall surface to the inside wall, the in-plane porosity decreases initially and, after at least one minimum area, it increases again in the outer wall area. It also discloses a method for removal of virus where an aqueous solution of protein is filtered using said membrane. When the membrane structure disclosed herein is briefly mentioned, it is a hollow fiber membrane where the pore size of the membrane wall becomes in the order of rough-dense-rough in the membrane thickness direction. It has been said to be suitable for the removal of virus in high efficiency and the recovery of protein in high permeation efficiency without denaturing protein that there are such an inclined structure and the specific average pore size. Although various macromolecular substances are exemplified as the material, the art disclosed in Patent Document 5 is substantially an art using regenerated cellulose and it is difficult to widely develop the art disclosed therein for many materials. Further, the disadvantages of a cellulose material were as mentioned already.